Automatic gain control loops are employed in a variety of applications to dynamically control the gain levels of an incoming signal. For example, receivers employ automatic gain control loops to adjust the gain of an incoming analog signal for maximizing the dynamic range of a subsequent analog-to-digital converter. In other words, the amplitude levels of the analog signal are adjusted to be as close to the conversion limits of the analog-to-digital converter, so to fully utilize the resolution of the analog-to-digital converter and hence reduce quantization noise. Automatic gain control loops can employ an automatic gain control component that compares an output signal of the analog-to-digital converter to a reference value that represents the square of the mean energy of the expected input signal in a worst case scenario. Error signals are generated based on the comparison over a plurality of output signal samples. The error signals are multiplied by a step size and accumulated to generate a gain word, which is a signal used to program a programmable or variable gain amplifier that controls the gain of the incoming signal.
Automatic gain control loops that employ a reference value that represents the square mean energy of a signal assume that the energy of the input signal is evenly distributed. However, if this assumption is not correct the operation of the automatic gain control loop will be severely affected. For example, a video signal conforming to National Television Standards Committee (NTSC) or Phase Alternate Line (PAL) has a synchronization portion and a data portion. The synchronization portion has a much stronger energy level than the data level. Furthermore, the data portion does not have evenly distributed energy. Therefore, an automatic gain control loop that employs a reference value that represents the square mean energy of a signal would be inappropriate for this type of signal. Another problem with an automatic gain control loop that employs a reference value that represents the square mean energy of a signal is that the reference value is calculated with respect to a worst case scenario, thus substantial headroom is incorporated into the calculation for a typical case. Therefore, the full dynamic range of the analog-to-digital converter is not employed. Yet another problem with this technique is that the convergence time of the loop is determined on a worst case scenario, which can be much longer than its actual convergence time.